Why is no-one selling/buying Ether (ETH) on Bisq?

A great to hear! Just make a request for adding an altcoin.
https://forum.bisq.io/t/how-to-add-your-favorite-altcoin/46

Officially it requires a PR for an address validation but if the altcoin sparks enough interest and we see that people are really willing to trade it we can make exceptions.
Most of the altcoins added never got traded at all, so that extra small hurdle with a PR should help to filter those out who are not very serious and professional.